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Stellar Feedback and Galaxy Evolution
Q. Daniel Wang
IRAC 8 micro
ACIS diffuse 0.5-2 keV
University of Massachusetts
Galaxy formation and evolution context
Toft et al. (2002); Muller & Bullock (2004)
The missing baryon problem
• Observed baryon mass in stars and the ISM
accounts for 1/3-1/2 of what is expected
from the gravitational mass of a galaxy.
• Where is the remaining baryon matter:
– In a hot gaseous galactic halo?
– Or having been pushed away?
• Both are related to the galactic energy
Forms of the galactic feedback
• AGNs (jets)
• Nuclear starbursts or superwinds
• Gradual energy inputs
– Galactic disks: massive star formation
– Galactic bulges: Type Ia SNe.
AGN feedback
• Centaurus A:
– D=3.5 Mpc
– Nearest radio-bright
– Lx(AGN) ~ 1042 erg/s
– Lx(diffuse) ~ 5x1038
• The total mechanical
energy output is not
• Most nearby galaxies
do not contain AGNs!
Karovska et al. 2002
Starburst feedback
0.5-2 keV
2-8 keV
Starbursts typically occur in
low-mass gas-rich galaxies
in the present Universe.
Feedback in normal galaxies: our Galaxy
binary Diffuse Background Map:
~50% of the background is thermal and local (z < 0.01)
The rest is mostly from faint AGNs (McCammon et al. 2002)
X-ray absorption line spectroscopy
X-ray binary
X-ray binary
Wang et al. 05, Yao & Wang 05/06,
Yao et al. 06/07
ROSAT all-sky survey
in the ¾-keV band
LMXB X1820-303
In the GC NGC 6624
– l, b = 2o.8, -8o
– Distance = 7.6 kpc 
tracing the global ISM
– 1 kpc away from the
Galactic plane  NHI
• Two radio pulsars in the
GC: DM  Ne
• Chandra observations:
– 15 ks LETG (Futamoto
et al. 2004)
– 21 ks HETG
LETG+HETG spectrum
Yao & Wang 2006, Yao et al. 2006
X-ray absorption line spectroscopy
along the X1820-303 sightline: Results
• Hot gas accounts for ~ 6% of the total O
column density
• Mean temperature T = 106.34 K
• O abundance:
– 0.3 (0.2-0.6) solar in neutral atomic gas
– 2.0 (0.8-3.6) solar in ionized gas
• Hot Ne/O =1.4(0.9-2.1) solar (90% confidence)
• Hot Fe/Ne = 0.9(0.4-2.0) solar
• Velocity dispersion 255 (165–369) km/s
Mrk 421
(Yao & Wang 2006)
•Joint-fit with the absorption lines
with the OVII and OVIII line
emission (McCammon et al. 2002)
•Model: n=n0e-z/hn; T=T0e-z/hT
 n=n0(T/T0), =hT/hn, L=hn/sin b
OVI 1032 A
Galactic global hot gas properties
• Non-isothermal:
– mean T ~ 106.3 K toward the inner region
~ 106.1 K at solar neighborhood
• Velocity dispersion from ~200 km/s to 80 km/s
• Consistent with solar abundance ratios
• A thick Galactic disk with a scale height 1-2 kpc,
~ the values of OVI absorbers and free electrons
• Enhanced hot gas around the Galactic bulge
• No evidence for a large-scale (r ~ 102 kpc) X-rayemitting/absorbing halo with an upper limit of
NH~1 x1019 cm-2
• But a large-scale hot halo is required to explain
HVCs: confinement and OVI line absorption!
Feedback from disk-wide star
Diffuse X-ray emission
compared with
HST/ACS images:
Red – H
Green – Optical R-band
Blue – 0.3-1.5 keV
• Lx(diffuse) ~ 4x1039 erg/s
• T1 ~ 106.3 K, T2 > 107.1 K
• Scale height ~ 2 kpc +
more distant blubs.
Li et al. (2008)
NGC 5775
Soria & Wu (2002)
Li et al. 2007
Extraplanar hot gas seen in
nearby galaxies
• At least two components of diffuse hot
– Disk – driven by massive star formation
– Bulge – heated primarily by Type-Ia SNe
• Characteristic extent and temperature
similar to the Galactic values
• No evidence for large-scale X-ray-emitting
galactic halos
Observations vs. simulations
• Little evidence for X-ray
emission or absorption from
IGM accretion.
No “overcooling” problem?
• Missing stellar energy
feedback, at least in earlytype spirals. Where does the
energy go?
Simulations by Toft et al. (2003)
NGC 4565 250
NGC 2613 304
NGC 5746 307
NGC 2841 317
NGC 4594 370
1-D Simulations of galaxy formation
with the stellar feedback
• Evolution of both dark and baryon matters
(with the final mass 1012 Msun)
• Initial bulge formation (5x1010 Msun) 
starburst  shock-heating and expanding
of gas
• Later Type Ia SNe  bulge wind/outflow,
maintaining a low-density high-T halo,
preventing a cooling flow
Tang & Wang 2007
1-D Simulations of galaxy formation
with the stellar feedback
• Both dark and baryon matters
evolve (with the final mass 1012
• A blastwave is initiated by the
SB (forming a 5x1010 Msun bulge)
and maintained by the Type Ia
SN feedback.
The IGM is heated beyond the
virial radius
The accretion can be stopped
and the shocked hot gas expands
The resultant low density allows
the bulge wind.
The wind can be shocked at a
large radius.
1-D Simulations of galaxy formation
with the stellar feedback
• If the specific energy of
the feedback is reduced
(e.g., because of massloading of the bulge wind),
the wind has then evolved
into a subsonic outflow.
• This outflow can be stable
and long-lasting
• Consistent with
observations of low Lx/LB
galaxies (relative higher Lx,
lower T, and more extended
than those predicted by a
supersonic wind.
Evolution of Baryons around galaxies
Total baryon
before the SB
baryon fraction
Total baryon
at present
Hot gas
• Galaxies such as the MW
evolves in a hot bubble
with a deficit of baryon
• This bubble explains the
lack of large-scale X-ray
• Bulge wind removes the
present stellar feedback.
• Results are sensitive to
the initial burst and to
the bulge/halo mass ratio
2-D simulations of galactic flows in M31
An ellipsoid bulge (q=0.6), a disk, and an NFW halo
3-D simulations of a galactic
Energy not dissipated
• Most of the energy is
in the bulk motion and
in waves
• Parallel, adaptive mesh
refinement FLASH code
• Finest refinement in
one octant down to 6 pc
• Stellar mass injection
and SNe, following
stellar light
• SN rate ~ 4x10-4 /yr
• Mass injection rate ~0.1
10x10x10 kpc3 box
density distribution
• Diffuse X-ray-emitting gas is strongly concentrated
toward galactic disks and bulges (< 20 kpc).
• Heating is mostly due to SNe. But the bulk of their
energy is not detected in X-ray near galactic
bulges/disks and is probably propagated into the halos.
• Feedback from a galactic bulge likely plays a key role in
galaxy evolution:
– Initial burst  heating and expansion of gas beyond the
virial radius
– Ongoing Type Ia SNe  keeping the gas from forming a
cooling flow
• Low n and high T are characteristics of the gaseous
• Mass-loaded subsonic outflows account for diffuse Xray emission from galactic bulges